One method of printing continuous tone images makes use of a thermal print head, heat sensitive media and a means for moving the media relative to the thermal print head. Most thermal print heads are a one-dimensional array of heating elements (often with integral driver IC's and shift registers) mounted on a ceramic substrate. The ceramic substrate is then mounted to a heat sink which may be metal. In systems utilizing this type of thermal print head it is often observed that the printing density is not uniform down the page, but rather increases with time even when the input is a constant (flat) field. This is due to the accumulation of heat in the material surrounding the line of heating elements. The problem of the increase in temperature of the material surrounding a line source of heat has been addressed in a book by Carslaw and Jaeger, entitled "Conduction of Heat in Solids", 2nd Ed. Page 339. The analysis disclosed in that book shows that the temperature at a distance R from a line source of heat of flux Q and radius a which is turned on at a time t= 0, is for large values of the time equal to: ##EQU1## where: T is the temperature
K is the thermal conductivity, PA1 k is the thermal diffusivity, and PA1 C is a constant.
A patent of interest for its teaching in this art is U.S. Pat. No. 4,688,051 entitled "Thermal Print Head Driving System" by T. Kawakami et al. The system of that patent supplies a predetermined number of driving pulses to each of a plurality of heat-producing elements arranged in a line. The pulse width of the driving pulses are controlled in accordance with the temperature at, or in the vicinity of, the heat-producing elements. This control maintains the density level of like tones at a substantially constant value. Also, in one aspect of that invention the number of driving pulses corresponding to a desired tone level, is altered in consideration of data collected from at least one of the preceding recording lines.
Another patent of interest is Japanese Patent No. 59-194874 entitled "Thermal Head Driver" by Mamoru Itou. The driver of that patent strives for a uniform printing density by controlling the spacing between constant pulse width current signals that are applied to heating resistors with the space between the pulses varying in accordance with the temperature of a substrate that forms part of the thermal print head. In this manner, as the temperature of the thermal print head increases the space between successive pulses is also increased due to the fact that less energy is needed to bring the heating elements up to a recording temperature. In a like manner, if the temperature of the head decreases the space between pulses is decreased in order to provide more heating energy to the heating elements.
Another patent of interest is Japanese Patent No. 60-72757 entitled "Thermal Recorder" by Kazushi Nagato. The recorder of that patent attempts to unify the image density in a screen of thermal printing by counting the number of lines from the starting point of printing to control the energized pulse width according to the line count. This technique counteracts the effect of having a cold head when the first lines of the image are being recorded versus having an extremely warm or hot head as the printer approaches the end of the page after having recorded many lines of image data.
Another patent of interest is Japanese Patent No. 60-90780 entitled "Thermal Printer" by Nobuaki Aoki. In that patent, printing pulses are controlled as a function of the number of pieces of data printed and the period of time corresponding to the printing. The system of that patent more specifically counts data for controlling the printing pulses during the printing of one piece of data and a timer counts the period of time elapsed between the end of printing of a first document and the start of printing for a subsequent document. The duration of time between printings is related to the cooling effect that will occur in a thermal print head from a start position corresponding to the count of dots existing for the previous printed page. This cooling will of course, if left uncompensated, cause a variance in the print density at the start of printing of the next document or image in the sequence.
From the foregoing it can be seen that control of the density of print by thermal printers is a problem that has been approached in a number of ways with the desired result being a uniform density down a printed page of data. The present invention is directed towards a solution to that problem.